(1) Field of the Invention
The invention relates to gas turbines and, more specifically, to liquid fuel injection systems for industrial gas turbines.
In particular, the present invention relates to a device for purging the systems that inject fuel into the gas turbines.
(2) Description of the Related Art
Gas turbines generally comprise an air intake system, a compressor with one or more compression stages having an air flow rate regulating device, an internal combustion system, an expansion turbine connected mechanically to the compressor, and a system for discharging the exhaust gases. Gas turbines are designed with combustion systems capable of injecting liquid fuel and/or gaseous fuel into the combustion system, via concentric injectors for example. However, certain gas turbines are capable of running alternately on liquid fuel and on gaseous fuel such as, for example, natural gas. In general, gas turbines burn each of the liquid and gaseous fuels alternately. Thus, when the gas turbine is burning a liquid fuel, the supply of gaseous fuel is cut off and when the gas turbine is burning a gaseous fuel, the supply of liquid fuel is cut off. During the switchover from liquid fuel to gaseous fuel, the pressure in the liquid fuel circuit drops gradually while the pressure in the gaseous fuel circuit gradually increases. This type of gas turbine entails a purge system auxiliary to the liquid fuel injection system to eliminate the liquid fuel present in the injectors of the combustion chamber and thus provide the injectors with a continuous cooling air stream. The purge system is generally switched into operation when the liquid fuel supply system starts or is shut down. This is because it is necessary to purge the liquid fuel supply circuit to prevent liquid fuel remaining in a high-temperature region near the combustion chamber which could lead to coking of the liquid fuel, namely to solidification of the liquid within the supply circuit and therefore progressive degradation of the operation of the gas turbine, blocking of valves, pipes, check valves.
One solution for purging the liquid fuel is to use compressed air or a liquid.
In this respect, reference may be made to document EP 0 949 454 which comprises a complex purge system comprising a purge air supply valve combined with a soft purge valve, a non-return check valve and a multi-orifice purge valve directing the purge air towards the injectors. However, this type of purge system proves complex.
Reference may also be made to document U.S. Pat. No. 6,438,963 which describes a purge system comprising a three-way valve to reduce the residual quantity of liquid fuel and effectively avoid combustion gases returning to the liquid fuel supply circuit and to the purge circuit.
However, because it is difficult to regulate the pressure of the purge air, the use of compressed air leads to sudden purges of liquid fuel into the combustion chamber, leading to a sudden increase in the power of the gas turbine.
In order to alleviate this disadvantage, the prior art proposes lengthening the purge time. However, lengthening the purge period means that the residual liquid fuel flow rate needs to remain constant during purging, which it rarely does. In addition, such a solution entails a highly complex purge control system that is difficult to install.